Solar Cell Structure Including A Silicon Carrier Containing A By-Pass Diode

ABSTRACT

A solar cell structure including a silicon carrier defining a front side and a back side, and including an N-type portion having an exposed portion on the front side of the carrier and a P-type portion having an exposed portion on the front side of the carrier, the N-type portion and the P-type portion defining a P-N junction, and a solar cell defining a front side and a back side, wherein the solar cell is connected to the front side of the carrier such that the back side of the solar cell is electrically coupled to the exposed portion of the N-type portion, and wherein the front side of the solar cell is electrically coupled to the exposed portion of the P-type portion.

GOVERNMENT CONTRACT

The inventions claimed herein were made with Government support underContract Number HR0011-08-C-0086 awarded by the Air Force. TheGovernment of the United States may have certain rights in the claimedinventions.

FIELD

This application relates to solar cell structures and, moreparticularly, to solar cell structures that include a by-pass diode.

BACKGROUND

To date, the highest conversion efficiencies achieved formonolithically-grown, series-connected, multi junction solar cells usessemiconductors with selected energy bandgaps, grown with atomic latticespacing closely matched to that of the growth substrate. The top twosubcells use Ga_(x)In_((1-x))P where x is around 0.5, giving bandgap1.85-1.95 eV for the top subcell. The next mid-cell usesIn_(x)Ga_((1-x))As with low In content, providing bandgap 1.38-1.43 eV.Higher efficiencies of the germanium (Ge) subcell is replaced by asubcell with bandgap between 0.95-1.05 eV of higher content indium ofIn_(x)Ga_((1-x))As. The solar spectrum is more effectively matched if afourth subcell with lower bandgap is included. These two lower subcellsuse In_(x)Ga_((1-x))As where x extends from 0.0 to 0.3. Thelattice-mismatch of these lower cells to Ge or GaAs growth substrate canbe between 2% and 5%, and can be potentially mitigated bylattice-grading or annealing schedules. The loss in output fromlattice-mismatch is offset by the better use of the solar spectrum andthe increased voltage. If the lower subcells are grown first,lattice-mismatch reduces the output of the top two cells, which provides75-85% of the multi junction cell output. To avoid these mismatcheffects on the top two subcells, inverted multi junction cells aregrown, with the top subcells are grown first and lattice closely matchedto the Ge or GaAs growth substrate. The lower subcells are grown withmismatched, but the improved energy gap selection can offset the loss inperformance resulting from the mismatch, and some mitigation ispossible.

A typical solar cell includes two semiconductor layers in facing contactat a semiconductor junction. When illuminated by the sun or otherwise,the solar cell produces a voltage across the semiconductor layers. Moreadvanced solar cells may include three or more semiconductor layers thatdefine multiple junctions.

The voltage and current output of a solar cell are limited by thematerials of construction and the active surface area of the solar cellstructure. Therefore, multiple solar cells are typically electricallyinterconnected, such as in series, to form a circuit that produceshigher voltages than are possible with a single solar cell. A typicalsolar panel is formed by electrically connecting several circuits, suchas in parallel or in series, to produce higher currents or highervoltages. A solar array may be formed as a combination of solar panels.Solar arrays are now used in space and terrestrial applications.

A circuit of solar cells works well when all of the solar cells areilluminated with generally the same illumination intensity. However, ifone of the solar cells is shaded while the others remain fullyilluminated, the shaded solar cell is subjected to a reverse-biascondition by the continuing voltage and current output of the remainingsolar cells.

By-pass diodes are used to protect against the damage arising during thereverse-bias condition. A by-pass diode blocks current when the solarcell is not reverse biased, but passes the impressed current when thesolar cell is reverse biased.

While certain techniques for incorporating by-pass diodes into solarcells are known, those skilled in the art continue to seek new ways ofincorporating by-pass diodes into solar cell structures.

SUMMARY

Disclosed is a carrier, attached after the growth of the cell layers.The carrier includes suitably patterned P-N junction, which withcontacts and bonding methods, provides a bypass diode to each multijunction cell structure. Several alternative solutions of connectingcontacts are given below. All of the advantages of an invertedmetamorphic cell are preserved and minimum degradation of performanceoccurs if shadowing occurs on the solar array.

In one aspect, the disclosed solar cell structure may include a siliconcarrier defining a front side and a back side, and including an N-typeportion having an exposed portion on the front side of the carrier and aP-type portion having an exposed portion on the front side of thecarrier, the N-type portion and the P-type portion defining a P-Njunction, and a solar cell defining a front side and a back side,wherein the solar cell is connected to the front side of the carriersuch that the back side of the solar cell is electrically coupled to theexposed portion of the N-type portion, and wherein the front side of thesolar cell is electrically coupled to the exposed portion of the P-typeportion.

In another aspect, the disclosed solar cell structure may include asilicon carrier defining a front side and a back side, and comprising anN-type portion having an exposed portion on the front side of thecarrier and a P-type portion having an exposed portion on the back sideof the carrier, the N-type portion and the P-type portion defining a P-Njunction, and a solar cell defining a front side and a back side,wherein the solar cell is connected to the front side of the carriersuch that the back side of the solar cell is electrically coupled to theexposed portion of the N-type portion, and a metallization layer thatelectrically couples the front side of the solar call to the exposedportion of the P-type portion.

In another aspect, disclosed is a method for assembling a solar cellstructure. The method may include the steps of (1) providing a siliconcarrier having a front side and a back side, (2) providing a solar cellhaving a front side and a back side, wherein the back side of the solarcell includes a first metallization layer connected thereto, (3) forminga P-N junction in the carrier such that the carrier includes a P-typeportion and an N-type portion, (4) forming a second metallization layeron the front side of the carrier, the second metallization layer beingelectrically coupled to the N-type portion, (5) connecting the firstmetallization layer to the second metallization layer to form ametal-to-metal bond between the solar cell and the carrier, and (6)electrically coupling the front side of the solar cell to the P-typeportion.

In yet another aspect, the disclosed method for assembling a solar cellstructure may include the steps of (1) providing a silicon carrierhaving a front side and a back side, (2) providing a solar cell having afront side and a back side, wherein the back side of the solar cellincludes a first metallization layer connected thereto, (3) forming aP-N junction in the carrier such that the carrier includes a P-typeportion exposed on the back side of said carrier and an N-type portionexposed on the front and the back sides of the carrier, (4) forming asecond metallization layer on the front side of the carrier, the secondmetallization layer being electrically coupled to the N-type portion,(5) connecting the first metallization layer to the second metallizationlayer to form a metal-to-metal bond between the solar cell and thecarrier, and (6) forming a third metallization layer that electricallycouples the front side of the solar cell to the P-type portion on theback side of the carrier.

Other aspects of the disclosed solar cell structure with integralby-pass diode and methods for assembling the same will become apparentfrom the following description, the accompanying drawings and theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view, in section, of a first aspect of thedisclosed solar cell structure with integral by-pass diode;

FIG. 2 is a side elevational view, in section, of a carrier of the solarcell structure of FIG. 1, shown with metallization and insulation layersapplied thereto;

FIG. 3 is a side elevational view, in section, of an invertedmetamorphic solar cell used to form the solar cell structure of FIG. 1;

FIG. 4 is a side elevational view, in section, of the invertedmetamorphic solar cell of FIG. 3, shown with an additional metallizationlayer;

FIG. 5 is a side elevational view, in section, of the invertedmetamorphic solar cell of FIG. 4 connected to the carrier of FIG. 2during the assembly of the solar cell structure of FIG. 1 in accordancewith one aspect of the disclosed method for assembling solar cellstructures;

FIG. 6 is a side elevational view, in section, of the solar cellstructure of FIG. 5, shown with a temporary substrate layer removedtherefrom;

FIG. 7 is a side elevational view, in section, of the solar cellstructure of FIG. 1 electrically coupled, in series, to another solarcell structure;

FIG. 8 is a side elevational view, in section, of a second aspect of thedisclosed solar cell structure with integral by-pass diode;

FIG. 9 is a side elevational view, in section, of a carrier of the solarcell structure of FIG. 8, shown with metallization layers;

FIG. 10 is a side elevational view, in section, of the invertedmetamorphic solar cell of FIG. 4 connected to the carrier of FIG. 9during the assembly of the solar cell structure of FIG. 8 in accordancewith another aspect of the disclosed method for assembling solar cellstructures;

FIG. 11 is a side elevational view, in section, of the solar cellstructure of FIG. 10, shown with a temporary substrate layer removedtherefrom;

FIG. 12 is a side elevational view, in section, of the solar cellstructure of FIG. 8 electrically coupled in series to another solar cellstructure;

FIG. 13 is a flowchart illustrating a method for assembling the solarcell structure of FIG. 1; and

FIG. 14 is a flowchart illustrating a method for assembling the solarcell structure of FIG. 8.

DETAILED DESCRIPTION

Referring to FIG. 1, a first aspect of the disclosed solar cellstructure, generally designated 100, may include a carrier 102 and asolar cell 104 connected to the carrier 102 by way of a metal-to-metalbond 106. The solar cell structure 100 may also include additionallayers, such as a front contact metallization layer 108 (e.g., a grid),an anti-reflecting coating layer 110 and a cover glass layer 112, amongother possible additional layers.

The solar cell 104 may include a front side 114 and a back side 116, andmay produce a voltage across the front side 114 and the back side 116when the front side 114 is illuminated (e.g., by the sun). The solarcell 104 may include multiple layers 118, 120, 122 of semiconductormaterial that define junctions 124, 126 therebetween. As discussedbelow, in one particular implementation, the solar cell 104 may be aninverted metamorphic solar cell.

The carrier 102 may include a portion of P-type semiconductor material128 and a portion of N-type semiconductor material 130, thereby defininga P-N junction 132 in the carrier 102. The front side 114 of the solarcell 104 may be electrically coupled to the portion of P-typesemiconductor material 128 of the carrier 102 (e.g., by way of aninterconnect 134) and the back side 116 of the solar cell 104 may beelectrically coupled to the portion of N-type semiconductor material 130of the carrier 102, thereby advantageously configuring the carrier 102as a diode, specifically, a by-pass diode.

In one particular implementation, the carrier 102 may be a generallyflat, planar structure, such as a wafer, and may include a front side136 and a back side 138. Both the P-type portion 128 and the N-typeportion 130 of the carrier 102 may be exposed on the front side 136 ofthe carrier 102. The back side 138 of the carrier 102 may include afirst layer of insulation 140. A second layer of insulation 142 may beprovided on a portion of the front side 136 of the carrier 102 toelectrically isolate the back side 116 of the solar cell 104 from theP-type portion 128 of the carrier 102.

Thus, as shown in FIG. 7, the solar cell structure 100 may be arrangedin an array with other like solar cell structures 100′ by usinginterconnects 134 in an all-front-contact configuration to electricallycouple the front side 114 of the solar cell 104 and the P-type portion128 of solar cell structure 100 with the back side 116 of the solar cell104 of an adjacent solar cell structure 100′.

One aspect of the disclosed method for forming solar cell structure 100is illustrated in FIG. 13 and generally designated 150. The method 150may begin with the separate formation of the carrier 102 (step 152) andthe solar cell 104 (step 154).

The carrier 102 (FIG. 1) may be formed from a silicon material, such assingle crystal silicon or polycrystalline silicon. At this point, thoseskilled in the art will appreciate that the use of a silicon material toform the carrier 102 may be advantageous for numerous reasons, includingthe low cost of silicon, the relative flexibility of silicon, the easeof forming P-N junctions in silicon, among other reasons.

The P-N junction 132 in the carrier 102 may be formed using anyavailable technique, including diffusion techniques and growthtechniques (e.g., chemical vapor deposition). In one example, thecarrier 102 may be formed by starting with a P-type silicon wafer anddiffusing a dopant relative to the P-type silicon wafer to form theportion of N-type semiconductor material 130 on the wafer and, thus, theP-N junction 132. In another example, the carrier 102 may be formed bystarting with an N-type silicon wafer and diffusing a dopant relative tothe N-type silicon wafer to form the portion of P-type semiconductormaterial 128 and, thus, the P-N junction 132.

Once the carrier 102 has been formed in step 152 (FIG. 13), the methodmay proceed to step 156 (FIG. 13), wherein metallization layers may beformed on the carrier 102. Specifically, referring to FIG. 2, a firstconnected pad 144 may be metalized on the front surface 136 of thecarrier 102 to electrically contact the N-type portion 130 of thecarrier 102 and a second connection pad 146 may be metalized on thefront surface 135 of the carrier 102 to electrically contact the P-typeportion 128 of the carrier 102. Insulation layer 142 may electricallyisolate the first connection pad 144 from the P-type portion 128 of thecarrier. Insulation layer 140 may be applied to the back side 138 of thecarrier.

With the formation, metallization and insulation of carrier 102complete, the carrier 102 may be cut to the desired shape and size, asshown in step 158 (FIG. 13).

At step 154 (FIG. 13), the solar cell 104 may be initially formed on atemporary substrate 160 with the front side 114 of the solar cell 104mated with the temporary substrate 160, as shown in FIG. 3. For example,the temporary substrate 160 may be germanium and the solar cell 104 maybe an inverted metamorphic solar cell that has been epitaxially grown onthe germanium substrate 160, as is known in the art. Other techniquesmay be used to form the solar cell 104 on the temporary substrate 160without departing from the scope of the present disclosure.

At step 162 (FIG. 13), a metallization layer 148 may be layered over theback side 116 of the solar cell 104, as shown in FIG. 4. Whilemetallization layers are well known in the art, examples of appropriatematerials for metallization layer 144 and metalized connection pads 146,148 include gold, silver and alloys thereof, among other possiblemetallization metals. With the metallization of the solar cell 104complete, the solar cell 104 and associated metallization layer 148 andtemporary substrate 160 may be cut to the desired size and shape, ashown in step 164 (FIG. 13).

Once the carrier 102 and the solar cell 104 have been separately formed,the method 150 (FIG. 13) may proceed to step 166. At step 166, themetallization layer 148 of the solar cell 104 may be connected to thefirst connection pad 144 of the carrier 102 to form the metal-to-metalbond 106 between the solar cell 104 and the carrier 102, as shown inFIG. 5. At step 168 (FIG. 13), the temporary carrier 160 may be removedfrom the solar cell 104, as shown in FIG. 6. The removal of thetemporary carrier 160 may be effected using any technique known oravailable in the art. At step 170 (FIG. 13), the additional layers, suchas the front contact metallization layer 108, the anti-reflectingcoating layer 110 and the cover glass layer 112, may be layered over thefront side 114 of the solar cell 104, as shown in FIG. 1. Then, at step172 (FIG. 13), the assembly may be heat treated (e.g., sintered) to formthe completed solar cell structure 100.

At step 174, the completed solar cell structure 100 may be electricallycoupled (e.g., in series) with other solar cell structures 100 by way ofthe interconnect 134, as shown in FIG. 7 and described above.

Referring to FIG. 8, a second aspect of the disclosed solar cellstructure, generally designated 200, may include a carrier 202 and asolar cell 204 connected to the carrier 202 by way of a metal-to-metalbond 206. The solar cell structure 200 may also include additionallayers, such as a front contact metallization layer 208 (e.g., a grid),an anti-reflecting coating layer 210 and a cover glass layer 212, amongother possible additional layers.

The solar cell 204 may include a front side 214 and a back side 216, andmay produce a voltage across the front side 214 and the back side 216when the front side 214 is illuminated. The solar cell 204 may includemultiple layers 218, 220, 222 of semiconductor material that definejunctions 224, 226 therebetween. Like solar cell 104, in one particularimplementation, solar cell 204 may be an inverted metamorphic solarcell.

The carrier 202 may include a front side 236, a back side 238, a portionof P-type semiconductor material 228 and a portion of N-typesemiconductor material 230. The P-type portion 228 and the N-typeportion 230 of the carrier define at least one P-N junction 232 in thecarrier 102.

Both the P-type portion 228 and the N-type portion 230 of the carrier202 may be exposed on the back side 238 of the carrier 202. The frontside 214 of the solar cell 204 may be electrically coupled to the P-typeportion 228 of the carrier 202 by way of a wrap-around metallizationlayer 234, which may be electrically isolated from the back side 216 ofthe solar cell 204 by way of an insulation layer 235. The back side 216of the solar cell 204 may be electrically coupled to the N-type portion230 of the carrier 202 by way of the metal-to-metal bond 206. Therefore,the carrier 202 may advantageously function as a by-pass diode.

Optionally, an additional metallization layer 237 (e.g., a connectionpad) may be provided on the back side 238 of the carrier and may beelectrically coupled to the N-type portion 230 of the carrier 202.

Thus, as shown in FIG. 12, the solar cell structure 200 may be arrangedin an array with other like solar cell structures 200′ by usinginterconnects 240 in an all-rear-contact configuration to electricallycouple the front side 214 of the solar cell 204 of solar cell structure200 with the back side 216 of the solar cell 204 of an adjacent solarcell structure 200′ by way of the carrier 202.

One aspect of the disclosed method for forming solar cell structure 200is illustrated in FIG. 14 and generally designated 250. The method 250may begin with the separate formation of the carrier 202 (step 252) andthe solar cell 204 (step 254).

At step 252, carrier 202 may be formed with a P-N junction 232 in thesame or similar manner as carrier 202. Once the carrier 202 has beenformed, the method may proceed to step 256 (FIG. 14), wherein a firstmetallization layer 244 (e.g., a first connection pad) may be formed onthe front side 236 of the carrier 202 and a second metallization layer237 (e.g., a second connection pad) may be formed on the back side 238of the carrier 202. Both the first 244 and the second 237 metallizationlayers may be electrically coupled to the N-type portion 230 of thecarrier 202.

With the formation and metallization of the carrier 202 complete, thecarrier 202 may be cut to the desired shape and size, as shown in step258.

Regarding step 254 (FIG. 14), the solar cell 204 may be initially formedon a temporary substrate 260 (FIG. 10) in a manner similar to solar cell104 shown in FIG. 3. Likewise, at step 262 (FIG. 14), a metallizationlayer 248 (FIG. 10) may be layered over the back side 216 of the solarcell 204. With the metallization of the solar cell 204 complete, thesolar cell 204 and associated metallization layer 248 and temporarysubstrate 260 may be cut to the desired size and shape, a shown in step264 (FIG. 14).

Once the carrier 202 and the solar cell 204 have been separately formed,the method 250 (FIG. 14) may proceed to step 266. At step 266, themetallization layer 248 of the solar cell 204 may be connected tometallization layer 244 of the carrier 202 to form the metal-to-metalbond 206 between the solar cell 204 and the carrier 202, as shown inFIG. 10. At step 268 (FIG. 14), the temporary carrier 260 may be removedfrom the solar cell 204, as shown in FIG. 11. At step 270 (FIG. 14),insulation layer 235 and wrap-around metallization layer 234 may beapplied to electrically couple the front side 214 of the solar cell 204with the exposed P-type portion 228 of the carrier 202, as shown in FIG.8. At step 272 (FIG. 14), the additional layers, such as the frontcontact metallization layer 208, the anti-reflecting coating layer 210and the cover glass layer 212, may be layered over the front side 214 ofthe solar cell 204, as shown in FIG. 8. Then, at step 274 (FIG. 14), theassembly may be heat treated (e.g., sintered) to form the completedsolar cell structure 200.

At step 276, the completed solar cell structure 200 may be electricallycoupled (e.g., in series) with other solar cell structures 200′ by wayof the interconnect 240, as shown in FIG. 12 and described above.

At this point, those skilled in the art will appreciate that thedisclosed solar cell structures and methods for assembling solar cellstructures provide advantages over the prior art. As one advantage, theby-pass diode is used as the manufacturing carrier to support the solarcell, thereby eliminating the need for separate carriers and separateby-pass diodes. As another advantage, the integration of the carrierwith the by-pass diode may eliminate the need for multipleinterconnects. As another advantage, the all-front-contact andall-rear-contact configurations simplify implementation and automationof assembly, and the all-front-contact configuration allows for the backside of the carrier to be fully insulated. In yet another advantage, useof a silicon-based carrier significantly reduces material andmanufacturing costs, and provides for relatively increased flexibilityof the solar cell structures.

Although various aspects of the disclosed solar cell structure withintegral by-pass diode have been shown and described, modifications mayoccur to those skilled in the art upon reading the specification. Thepresent application includes such modifications and is limited only bythe scope of the claims.

1. A solar cell structure comprising: a silicon carrier defining a frontside and a back side, and comprising an N-type portion having an exposedportion on said front side of said carrier and a P-type portion havingan exposed portion on said front side of said carrier, said N-typeportion and said P-type portion defining a P-N junction; and a solarcell defining a front side and a back side, wherein said solar cell isconnected to said front side of said carrier such that said back side ofsaid solar cell is electrically coupled to said exposed portion of saidN-type portion, and wherein said front side of said solar cell iselectrically coupled to said exposed portion of said P-type portion. 2.The solar cell structure of claim 1 wherein said carrier is formed fromsingle crystal silicon.
 3. The solar cell structure of claim 1 whereinsaid solar cell comprises an inverted metamorphic solar cell.
 4. Thesolar cell structure of claim 1 wherein said solar cell is connected tosaid front side of said carrier by a metal-to-metal bond.
 5. The solarcell structure of claim 1 further comprising an interconnect thatelectrically couples said front side of said solar cell to said exposedportion of said P-type portion.
 6. The solar cell structure of claim 1further comprising a first metallization layer positioned over saidexposed portion of said N-type portion and a second metallization layerpositioned over said exposed portion of said P-type portion.
 7. Thesolar cell structure of claim 1 wherein said back side of said carrierincludes an insulation layer connected thereto.
 8. An array of the solarcell structures of claim 1 interconnected in series.
 9. A solar cellstructure comprising: a silicon carrier defining a front side and a backside, and comprising an N-type portion having an exposed portion on saidfront side of said carrier and a P-type portion having an exposedportion on said back side of said carrier, said N-type portion and saidP-type portion defining a P-N junction; and a solar cell defining afront side and a back side, wherein said solar cell is connected to saidfront side of said carrier such that said back side of said solar cellis electrically coupled to said exposed portion of said N-type portion;and a metallization layer that electrically couples said front side ofsaid solar call to said exposed portion of said P-type portion.
 10. Thesolar cell structure of claim 9 wherein said carrier is formed fromsingle crystal silicon.
 11. The solar cell structure of claim 9 whereinsaid solar cell comprises an inverted metamorphic solar cell.
 12. Thesolar cell structure of claim 9 wherein said solar cell is connected tosaid front side of said carrier by a metal-to-metal bond.
 13. The solarcell structure of claim 9 further comprising an insulation layerpositioned to electrically isolate said metallization layer from saidback side of said solar cell and said N-type portion.
 14. The solar cellstructure of claim 9 wherein said N-type portion includes a secondexposed portion on said back side of said carrier, and wherein a secondmetallization layer is positioned over said second exposed portion. 15.The solar cell structure of claim 14 wherein said metallization layer iselectrically coupled to a first adjacent solar cell structure by way ofa first interconnect and said second metallization layer is electricallycoupled to a second adjacent solar cell structure by way of a secondinterconnect.
 16. An array of the solar cell structures of claim 9interconnected in series.
 17. A method for assembling a solar cellstructure comprising the steps of: providing a silicon carrier having afront side and a back side; providing a solar cell having a front sideand a back side, wherein said back side of said solar cell includes afirst metallization layer connected thereto; forming a P-N junction insaid carrier such that said carrier includes a P-type portion and anN-type portion; forming a second metallization layer on said front sideof said carrier, said second metallization layer being electricallycoupled to said N-type portion; connecting said first metallizationlayer to said second metallization layer to form a metal-to-metal bondtherebetween; and electrically coupling said front side of said solarcell to said P-type portion.
 18. The method of claim 17 wherein saidfront side of said solar cell is electrically coupled to said P-typeportion on said front side of said carrier.
 19. The method of claim 17wherein said front side of said solar cell includes a temporarysubstrate connected thereto, and wherein said method further comprisesthe step of removing said temporary substrate from said front side ofsaid solar cell after said connecting step.
 20. A method for assemblinga solar cell structure comprising the steps of: providing a siliconcarrier having a front side and a back side; providing a solar cellhaving a front side and a back side, wherein said front side of saidsolar cell includes a temporary substrate connected thereto, and whereinsaid back side of said solar cell includes a first metallization layerconnected thereto; forming a P-N junction in said carrier such that saidcarrier includes a P-type portion exposed on said back side of saidcarrier and an N-type portion exposed on said front and said back sidesof said carrier; forming a second metallization layer on said front sideof said carrier, said second metallization layer being electricallycoupled to said N-type portion; connecting said first metallizationlayer to said second metallization layer to form a metal-to-metal bondbetween said solar cell and said carrier; after said connecting step,removing said temporary substrate from said front side of said solarcell; and forming a third metallization layer that electrically couplessaid front side of said solar cell to said P-type portion.
 21. Themethod of claim 20 wherein said third metallization layer iselectrically isolated from said back side of said solar cell and saidN-type portion.